PmodADC

Overview

A fully discrete, 14-bit, 41kHz, Successive-Approximation Analog-to-Digital converter, controlled by an iCE40 FPGA. Audio can be streamed to and from the PC in real time via USB. The converter interfaces with an iCEbreaker FPGA board using a standard Pmod connector - which is where it takes its name from.

Because a 14-bit R2R DAC sits at the core of this design, I also built a second dedicated audio DAC board: the PmodDAC. Details of this design are also included below.

This project is a continuation of my previous SAR-ADC, which was a much simpler and slower converter controlled by a state machine implemented in discrete 74-series logic gates.

PmodADC

Below is a short overview of the different blocks of the PmodADC and PmodDAC schematics.

For the full schematics, see the links section at the end of this page.

The schematic snippets below can be enlarged by clicking on them.

Frontend + Sample & Hold

The input stage takes care of buffering, amplification, filtering, and biasing, before feeding a sample & hold amplifier.

First, the input signal (which is usually an audio signal with less than 1Vpp) is AC-coupled, buffered, and may be amplified with a variable gain of up to 20db.

A second order 20kHz Sallen-Key low-pass filter to reduce aliasing is in turn followed by a passive biasing network. The bias point is derived from the same 5V reference that also feeds the R2R DAC. A small trimmer provides a few millivolts of bias-point adjustment.

The frontend also includes some jumpers to reconfigure or bypass the signal path. This is useful as it provides a DC-coupled connection to the ADC during characterization.

The sample & hold stage is a LF398 amplifier implemented as recommended. The actual sampling capacitor is a 10nF C0G ceramic. It too can be bypassed by some configuration jumpers. Oscilloscope traces showing the operation of the sample & hold amplifier are included below.

DAC + Comparator

A REF3450 5V reference sets the full scale voltage of the DAC at the center of the converter, hence providing the reference voltage for the whole ADC.

The DAC is a standard discrete 14-bit R2R design. It is controlled by the FPGA via two shift registers which also handle level shifting of the 3.3V FPGA signals to 5V. The shift register outputs are buffered by two octal logic buffers which are powered directly from the 5V reference.

The DAC and sample & hold amplifier feed a LT1711 comparator, chosen for it's low offset voltage and speed.

Power

I originally wanted the board to be powered solely by the 3.3V provided by the Pmod Connector, so I designed and implemented the above power architecture:

The 3.3V input is first boosted to 10V, and then regulated to the 5V and 9V rails needed using two LDO regulators. The 9V rail is required because the LF398 Sample & Hold amplifier needs quite some supply voltage overhead to work up to 5V. To generate the negative -5V rail, the 3.3V input is first inverted to -6V, and then regulated using another LDO regulator.

Annoyingly, I overlooked that the 3.3V rail on the iCEbreaker board is produced by a small SOT-23-5 sized LDO regulator that is not able to continuously provide the 50-100mA that both boards draw. I ended up chopping out the 3.3V pins of the Pmod Connector and powering both boards from a lab supply.

PmodDAC

As mentioned above, I also built a 14-bit, 41kHz Audio DAC board as most of the components required had already been designed.

The power supply, FPGA interface, and R2R DAC are identical to the ADC above. Only the output stage differs:

First the DAC output is buffered to prevent errors due to loading. The signal is then AC-coupled, and may be attenuated using a potentiometer. This is again followed by a buffer.

Because there were extra op amps available anyway, I decided to include two Sallen-Key low pass filters, but one may be bypassed using configuration jumpers.

Construction

The board was assembled at home using standard methods. Below are some pictures of the process.

Solder paste being applied using a stencil.

Solder paste on the board.

Components placed before soldering.

A video of the 0402 R2R DAC Resistors being re-flowed on an early revision of the board:

Demonstration

The quality of both the ADC and DAC were actually much better than I expected.

When used to convert audio signals they both impart some conversion artifacts and add a very noticeable noise floor, but the audio remains of very acceptable quality.

As a quick demonstration, below is a video of music coming being recorded by the ADC and played back by the DAC:

As a more direct comparison, the table below contains short music clips as they are, after being recorded by the ADC, and after playback by the DAC respectively.

All music used is by Kevin MacLeod and is available under the Creative Commons 4.0 license, see here.

Characterization

I was very fortunate to be given the opportunity do some basic characterization of my ADC and DAC design at the Center for Project-based learning at ETHZ.

ADC Linearity

All ADC Linearity tests where performed with the input stage bypassed to DC couple the Keysight B2902A Source Meter used as test voltage source.

The absolute deviation from the ideal ADC transfer function is shown in the left plot below. When adjusting for the rather significant gain and offset errors, the non-linearities are at most 40LSB, as can be seen in the right plot.

The most likely sources for these errors are DAC non-linearities, since the DAC linearity measurements (below) feature a similar magnitude.

ADC Noise

With the input AC-coupled but shorted, the ADC will read as is show in the plot below.

The readings feature a standard deviation of 3.43

ADC Frequency Response

A rough measurement of the frequency-response of the ADC was taken by sampling single-tone signals from a Keysight 33600 function generator and examining the recorded signal amplitude.

I was surprised by this measurement: Both the input high-pass/AC-coupling and antialiasing low-pass seem to perform exactly as designed. The passband ripple is less than 0.5dB.

ADC SNR and THD

Some very rough SNR and THD measurements yield the following:

DAC Linearity

Measuring the DAC linearity again using the Keysight B2902A Source meter yielded the following:

Because this DAC is identical to the one at the core of the ADC, and the non-linearities are very similar in magnitude to those seen on the ADC, it is likely that DAC non-idealities are large source of the ADC errors.

DAC THD

Some very rough THD measurement yields the following:

A big thank-you again to Dr. Vogt and the ETH PBL Center for giving me the opportunity to put my design through its paces.

Ideas + Possible Improvements

The logical next step would probably be to integrate the ADC, DAC, FPGA and USB interface onto the same board. This could simplify the somewhat convoluted power architecture, possibly reducing noise.

This would also enable me to select the external oscillator FPGA to give more flexibility in sampling rate selection. A more standard frequency (44.1 kHz) would make sense. The FPGA could also be directly connected to the ADC: Skipping the shift registers and their high-speed serial communication would probably also yield noise improvements.

Another interesting idea would be to implement a charge-redistribution ADC from discrete components. I would presume that this would not be as straight forward, but definitely worth a try. An integrating/multislope ADC would also make for a fun challenge.

Links

• 📁 Repo
• 📦 Production Files
• PmodADC
  • 📝 Schematic
  • 📃 Interactive BOM
• PmodDAC
  • 📝 Schematic
  • 📃 Interactive BOM

Other Notes

The music used during all demonstration was:

Protofunk by Kevin MacLeod
Link: https://incompetech.filmmusic.io/song/4247-protofunk
License: http://creativecommons.org/licenses/by/4.0/

Whiskey on the Mississippi by Kevin MacLeod
Link: https://incompetech.filmmusic.io/song/4624-whiskey-on-the-mississippi
License: http://creativecommons.org/licenses/by/4.0/

New Hero In Town by Kevin MacLeod
Link: https://incompetech.filmmusic.io/song/5742-new-hero-in-town
License: http://creativecommons.org/licenses/by/4.0/

The Parting by Kevin MacLeod
Link: https://incompetech.filmmusic.io/song/4501-the-parting
License: http://creativecommons.org/licenses/by/4.0/